I ended up with an arbitrary choice between two ways to place the 8s in the first two columns. Choosing the opposite to the solution shown swaps a bunch of 3s with 6s elsewhere, giving a second solution:

Doh! You are absolutely right. Sorry for the false alarm. I'm very happy to be wrong though, I really appreciate the varied but fair challenge of your puzzles.

This is what I get for trying to save trees, ink and the terrible noise from my printer by copying the grid by hand I thought I'd double-checked too, but I missed the clue tucked under that stair-step. The one snag of squiggly grids is that it seems to be trickier to spot asymmetry when checking one's copying.